1. Field of the Invention
The present invention relates to a method and apparatus for substantially reducing cross-polarized radiation in offset reflector antennas and, more particularly, to method and apparatus for substantially reducing cross-polarized radiation in offset reflector antennas by substantially cancelling the polarization rotation produced in a large offset reflector by the opposite prerotation of the incident feed radiation via a polarization grid having parallel spaced-apart reflecting elements and disposed at a predetermined angle to a plane normal to the feed axis.
2. Description of the Prior Art
Cross-polarized radiation from an offset reflector is often regarded as a blemish on an otherwise excellent antenna which offers both low sidelobe level and good impedance matching. Although the cross polarization can be minimized using a large effective F/D ratio, the corresponding requirements of small offset angle and large feed aperture are not always convenient in applications.
Various techniques have been devised to substantially reduce cross-polarized radiation. One technique is to detect the cross-polarized radiation component and convert such component into a suitable control signal to minimize the effect of cross polarization. In this regard see, for instance, U.S. Pat. Nos. 3,044,062 issued to M. Katzin on July 10, 1962 and 3,453,622 issued to L. J. McKesson on July 1, 1969.
U.S. Pat. No. 3,363,252 issued to P. S. Hacker on Jan. 9, 1968 discloses an arrangement which mounts cross-polarization suppressor fins in a longitudinal direction on the external sides of an antenna feedline or feedhorn to cancel cross-polarization vectors and maintain the electric vectors parallel to the sides of the feedlines or feedhorn and perpendicular to the fins.
U.S. Pat. No. 3,914,764 issued to E. A. Ohm on Oct. 21, 1975 discloses that linearly polarized transmitted waves experience polarization rotation and polarization conversion effects especially in the ionosphere. The Ohm arrangement uses microwave components to transform the two varying elliptically polarized waves into replicas of the rotated transmitted waves and then uses a conventional polarization rotator to align the waves in their originally transmitted directions.